(1) Field of the Invention
The invention relates to a method of fabricating both high and low voltage CMOS transistors in the fabrication of integrated circuits, and more particularly, to a method of fabricating both high and low voltage CMOS transistors while reducing shallow trench isolation (STI) oxide loss in the fabrication of integrated circuits.
(2) Description of the Prior Art
With the advent of large scale integration, many of the integrated circuits formed on a semiconductor substrate comprise several circuit functions on a single chip. For example, electrically programmable random access memory (EPROM) devices and FLASH EPROM devices include both memory devices and logic devices on a single chip. To optimize the devices and improve performance, it is desirable in the industry to provide logic devices having both thick and thin gate oxide layers thereunder. For example, typically a thin gate oxide is used in low voltage logic circuits to enhance device performance, while a thicker gate oxide is required for the high voltage transistors.
Typically, the tunnel oxide, floating gate, and ONO (oxide/nitride/oxide) layer are formed in the memory cell area first. Then, the thick gate oxide is grown. The thick gate oxide is removed in the low voltage area and a thin gate oxide is grown there. Unfortunately, removal of the thick gate oxide in the low voltage area will cause oxide loss from an underlying shallow trench isolation (STI) region, especially in the corner area. This will degrade electrical performance of the device. It is desired to find a way to form gate oxide layers of differing thicknesses without causing oxide loss in underlying STI regions.
U.S. Pat. No. 5,440,158 to Sung-Ma shows a method for forming a dual floating gate device. U.S. Pat. No. 5,723,355 to Chang et al teaches forming oxides having three different thicknesses. The method uses photoresist and etch back steps to form the three different thicknesses. U.S. Pat. No. 5,576,226 to Hwang teaches a method of forming oxide of two different thicknesses by implanting halogen ions into the portion of the substrate over which a thicker oxide layer is to be formed. U.S. Pat. No. 5,668,035 to Fang et al teaches growing a thick oxide overall, removing the thick oxide in an area, and then growing a thin oxide in that area.